Flocculants are reagents which are added to suspensions of solids to cause the solids to agglomerate (or flocculate) into larger particle sizes which settle and/or dewater more efficiently than in their original state. Polymeric water soluble flocculants may be nonionic, anionic, cationic or amphoteric. The particular type of flocculant to choose in a given application may depend upon the nature of the surface of the suspended solids, the pH of the aqueous slurry, and the desirable or undesirable side effects.
Approximately 5 to 6 million tons of kaolinitic clay are mined, processed and shipped from the southeastern United States per year. The clay is usually mined by the open pit method. It is then slurried in water at about 20-30% solids, dispersed with various combinations of inorganic and organic dispersants to maintain a low viscosity, and processed. Processing includes removal of iron and other magnetic impurities, removal of sand and silt, bleaching for whiteness and brightness, and, finally, dewatering. Dewatering is commonly accomplished by rotary vacuum filters although other methods of dewatering such as pre-thickening, centrifugation, and filtration on pressure filters are feasible. The filter cake obtained from the rotary vacuum filters is, in the present state of the art, usually about 50% to about 60% solids.
Also in the present state of the art, a portion of the filter cake is typically spray dried and blended back with the remaining filter cake to obtain an aqueous slurry containing 70% solids or more. Additional dispersants may be added to the 70% solids slurry with mixing as needed to obtain a final Brookfield viscosity of less than 1000 cps, and preferably less than 500 cps. This low viscosity, 70% solids slurry is then shipped in bulk to the customer.
My U.S. Pat. Nos. 4.647,382 and 4,741,838 disclose the use of certain anionic polymers of 2-acrylamido-2-methyl propane sulfonic acid AMPS.RTM. or their water soluble salts as flocculants which improve the dewatering of mineral slurries and which also enable the ready redispersal of the filter cake to a low viscosity, high solids slurry. My copending application filed simultaneously herewith entitled "Preparation of High Solids Mineral Slurries" discloses the use of certain polymers of dicyandiamide and formaldehyde and certain derivatives thereof as flocculants to improve the dewatering of mineral slurries and, quite surprisingly, also to enable the ready redispersal of the filter cake to a low viscosity, high solids slurry. FNT .RTM. Registered Trademark of Lubrizol Corp., Cleveland, Ohio
Certain organic cationic polymers are known to be effective flocculants and, in fact, are widely used as such in water, waste water and process water treatment. Typical of these products are aminomethylated polyacrylamides (AMPAM's) such as those described in U.S. Pat. No. 3,323,979, polyquaternary amines (PQA's) such as those described in U.S. Pat. Nos. Re. 28,807 and 28,808, and polydiallyldimethyl ammonium chlorides (DADMAC's or DMDAAC's) prepared according to U.S. Pat. No. 3,288,770.
Logically these same organic cationic polymers should effectively flocculate mineral slurries such as kaolinitic clays and, in fact, they are effective flocculants and are widely known to be effective for these applications. In particular, products of this type are commonly used in the clay industry in the clarification of magnet wash water wherein the solids are disposed of and are not returned to a process stream. However, these products have not been employed in treatment of any mineral slurry wherein the solids remain in the main-line process and wherein the mineral solids must subsequently be redispersed to a high solids, low viscosity slurry. Once a mineral slurry such as kaolin clay, calcium carbonate or calcium sulfate has been flocculated with one or more of these state-of-the-art cationic flocculants, the floc will not break down. In fact, it has been impossible to satisfactorily redisperse mineral slurries so treated by conventional means. Thus, the possible use of these cationic polymers for the applications described in the present application, specifically the flocculation, concentration and redispersal of high solids mineral slurries, has been precluded by the above noted devastating side effects.
Water soluble and dispersible polymers of melamine formaldehyde are widely used in the paper industry primarily as wet strength agents. For example, melamine/formaldehyde resins prepared from one mole of melamine reacted with up to three moles of formaldehyde are approved by the Food and Drug Administration for use in molded articles used in food packaging. Similar melamine formaldehyde polymers are also approved for use in paper and paperboard products to improve wet strength. They have also shown some limited utility as flocculants for low solids aqueous suspensions, such as surface water clarification. Commercially available products of this type include ChemLink "Perchem 525" and American Cyanamid's Magnifloc 515. However, prior to the present invention there was (1) no reason to expect these products to function as flocculants in a high solids aqueous suspensions such as the mineral slurries which are the subject of this invention and (2) absolutely no reason to expect that such cationic polymers would not exert a devastating effect on the viscosity of the redispersed slurries after dewatering.